This invention is directed to thermoplastic compositions having improved X-ray contrast, and in particular thermoplastic compositions comprising polysiloxane-polycarbonate and X-ray contrast agents, methods of making and of improving X-ray contrast in thermoplastics, and articles prepared therefrom.
Polycarbonate molds are commonly used in chocolate production. Such molds are generally made by plastic injection mold-making which involves high pressure injection of a polycarbonate resin around a metal master. Such polycarbonate molds have a long useful life and are very strong, and are desirable for both ease of use and for making high gloss chocolates. Other types of molds such as those made of silicone rubber or thermoformed plastic are also used for a limited set of applications. Polysiloxane-polycarbonate copolymers are also used in chocolate molds, primarily because of their exceptional release characteristics and chemical and thermal stability, as well as their ability to provide high gloss surfaces.
Metal detectors are generally used in the manufacturing process to detect metallic contaminants which typically arise from wear and tear of machines resulting from routine use, to ensure that chocolates prepared from them are of the highest quality. However, they are not capable of detecting any contaminants coming from plastic components (such as a chocolate mold) in the manufacturing process. X-ray detectors can be used to detect plastics; however, polycarbonate and typical compositions, such as for example those useful for preparing chocolate molds, are transparent to X-rays. Elements having higher atomic numbers than carbon have to be introduced into polycarbonate, either in the form of additives (e.g., glass, pigments, etc.) or incorporated into the polymer chain, to enable articles prepared from the polycarbonate to show any level of X-ray contrast. Typically addition of glass or other inorganic fillers leads to loss of gloss and low transparency as well as reduction in flow and impact properties of polycarbonate. This is not acceptable in the case of polysiloxane-polycarbonate matrices where low temperature ductility in combination with good flow is important.